This invention relates to adhesively sealed electronic packages for encapsulating integrated circuit devices. More particularly, the invention relates to a polymer plug for sealing a vent hole.
Electronic packages provide mechanical and environmental protection for integrated circuit devices such as silicon based semiconductor circuits. The electronic package also provides a means for electrical signals to travel between the encapsulated device and external circuitry.
One electronic package includes separate base and cover components defining a cavity. An integrated circuit device is housed within the cavity and a leadframe disposed between and bonded to both the base and the cover. The inner lead portions of the leadframe are within the cavity and the outer lead portions extend beyond the perimeter of the package. Small diameter wires electrically interconnect the inner leads to input/output pads on an electrically active face of the integrated circuit device. The outer leads are then electrically connected to a printed circuit board or other external circuit.
The base and cover are bonded together with a dielectric such as a low temperature sealing glass or a polymer adhesive. One common sealant is an epoxy sheet preform as disclosed in U.S. Pat. Nos. 4,105,861 to Hascoe, 4,594,770 to Butt, and 4,939,316 to Mahulikar. The base and cover components may be any suitable material such as a polymer, ceramic or metal. High thermal conductivity metals such as copper and aluminum are favored. These metals facilitate the removal of heat from the encased semiconductor device. Aluminum alloys provide the additional benefits of being lightweight and are capable of having a dielectric coating formed by anodization as disclosed in U.S. Pat. No. 4,939,316, which is incorporated by reference herein.
A preferred means for sealing the base and cover is a thermosetting polymer such as an epoxy. When the epoxy cures, ionic contaminants such as chlorides (Cl.sup.-) are generated. The ionic contaminants deposit on the thin bond wires and the electrically active face of the semiconductor device. In the presence of moisture, the ionic contaminants form dilute acids such as HC1, which corrode package components.
To avoid the accumulation of ionic contamination, a vent hole is disclosed in U.S. Pat. Nos. 4,897,508 and 5,013,871, both to Mahulikar and both incorporated by reference herein. The vent hole allows both ionic contaminants and water vapor to escape the package cavity during cure. The vent hole is subsequently sealed by epoxy bonding a countersunk metallic plug over the vent hole.
A metallic plug to seal the vent hole is satisfactory when a package has an overall thickness in the range of 3-4 millimeters (0.118-0.157 inch). The current trend is to thinner packages, on the order of less than 2 mm (0.078 in.) thick. These packages are lighter and take up less space than conventional packages. Both weight and real estate are important considerations in commercial, aerospace and military applications. Also, with a thinner package, the integrated circuit device is closer to a printed circuit board or external heat sink, enhancing the dissipation of heat from the device.
One way to form thinner packages is by reducing the thickness of the epoxy seal. Epoxy preforms are described in U.S. Pat. No. 5,013,871 as window frame shaped structures stamped from an epoxy sheet. To facilitate handling, the sheet generally has a thickness of about 0.38 mm (0.015 in.). Two preforms are utilized, one between the base and the leadframe, the other between the leadframe and the cover. The two epoxy sheets contribute 0.76 mm (0.30 in.) to package thickness. Replacing the preform sheets with a dispensed liquid adhesive as disclosed in U.S. patent application Ser. No. 07/987,867 to Liang, filed Dec. 9, 1992, and incorporated by reference herein, can reduce the total epoxy thickness to about 0.1 mm (0.004 in.).
Another way to reduce the thickness of the electronic package is to reduce the thickness of the base and cover components. For example, it is proposed to reduce the thickness of the cover component from 0.635 mm (0.025 in.) to 0.25 mm (0.010 in.). Since a vent hole is still required, this process is not amenable to the current practice of adhesively bonding a metallic plug over a countersunk vent hole. The metallic plug would be extremely thin, prone to bending, and difficult to handle. Further more, because of the thinness of the cover, a countersunk vent hole can not be reliably formed by stamping or machining.
The present invention is therefore drawn to a vent hole plug which does not have the disadvantages of the prior art and is amenable to use in a thin package.